1. Field of the Invention
The invention relates to air filtering fabrics and methods of their manufacture and more particularly relates to the manufacture of non-woven filter fabrics from thermoplastic, synthetic polymer, staple fibers and to the filter fabrics so made.
2. Brief Description of the Prior Art
The art is replete with descriptions of processes for preparing air filter fabrics from webs and batts of non-woven, thermoplastic, polymeric resin staple fibers. For example, one method of the prior art has been to compress non-woven webs of thermoplastic fibers under elevated temperatures and pressures to bond the fibers together at cross-over points of the fibers, i.e.; where the fibers touch and are pressed into contact with each other. Although this technique provides a filter fabric with unit structural integrity, it also results in a weakening of a high percentage of the individual fibers and a significant reduction in air permeability due to the occlusion or partial occlusion of a percentage of the pore openings. The difficulty may be appreciated by referring to FIG. 1, a magnified view of the fibrous structure in a prior art polypropylene fibrous filter made by the above described prior art method. As shown in FIG. 1, a high percentage of the thermoplastic fibers 10, which are normally substantially round, are flattened, pressed thin (and thereby weakened), disformed and reduce the intervening fabric pores. The fibers 10 may be aptly described as "collapsed."
Another prior art method of making filter fabrics has comprised impregnating the web of non-woven fibers with a curable resin 14 and curing the resin in place. As shown in FIG. 2, a magnified view of the polypropylene fibrous structure obtained by this latter method, this procedure will occlude at least partially the pores 15 in the fabric structure between fibers 12, thereby reducing air permeability. Although the procedure makes a strong filter fabric, it also tends to reduce the flexibility of the fabric, as those skilled in the art will appreciate, because of the increased thickness of the fiber unit and rigidification due to the cured coating.
By comparison, the method of the present invention bonds (in fact fuses) the individual staple fibers together at cross-over points without altering the fundamental shape, geometry or configuration of the basic fiber unit to any significant degree. There is no significant reduction in pore sizes or air permeability of the product filter fabric. The fabric retains a high degree of flexibility (enough to render the fabric particularly useful as an endless moving filter belt). In addition, the filter fabric of the invention gains structural integrity which was the objective of the previously described prior art methods. This structural integrity in the filter fabrics of the invention assures that they will avoid compacting of the filter mass under the impact of air flow during use. This is usually a cumulative impact under the maximum velocity of air flow over a period of time and results in reduced air permeability over a period of time. There is no weakening of individual fiber strength when the filter fabrics are made according to the process of the invention. Photomicrographs have shown that, for example, polypropylene fiber structures in the products of the process of the invention are essentially unchanged as shown in FIG. 3, a magnified view of a fibrous filter fabric of the invention. That is, the polypropylene fibers 16 remain essentially unchanged in configuration and dimension. There may be a slight bulging at times of the fiber. At the cross-over point 18 between two fibers 16, there is a fusion and blending of the fibers as more clearly seen in FIG. 4, a cross-sectional view along lines 4--4 of FIG. 3. This structure remains maximum fiber strength, maximum isotropic flexibility and maximum air permeability while gaining the added structural integrity found when the fibers are fused together at cross-over points.
The preferred filter fabrics of the invention are composites having different properties in the upper and lower surfaces with a gradient of these properties from top to bottom. More specifically, the preferred filter fabric composites of the invention have a smooth, lint free, release surface having controlled air permeability and maximum dry particle filter efficiency. The bottom surface of these fabrics of the invention on the other hand will have a high degree of abrasion resistance, air permeability and flexibility to enable their use in endless belt utilizations. The composite filter fabrics of the invention also exhibit advantageous tensile strengths through their entire thickness and transversely. These preferred composite filter fabrics are particularly useful as dry particle filters for use in moving filter belts, to be mounted in painting booths employing powder coating production lines, to collect and return excess depositions of electrostatic dry powder such as polyester, epoxy, glass and acrylic pigment powders for electrostatic deposition. The preferred belts of the invention supply the desired surface collection properties, i.e.; little penetration of fabric with small (5 to 20 micron) particles but with high air permeability for the return of collected particles to the electrostatic guns. These endless belts employing the filter fabrics of the invention are advantageous in that they reduce maintenance costs and allow for deep cleaning of the filter fabric when recovery of the dry particles is desired. The belts of the invention track well and show improved abrasion resistance and strength, particularly along the peripheral edges of the belts.
The filter fabrics of the invention exhibit long wear characteristics, low maintenance costs and do not tend to occlude or plug up with collected materials.